Been trying the same root partition. But I've been having other problems to lead to the finish.... Tried 3 times so far (get annoyed and ignore it for a few weeks, then try again).

First time:
--- Installed redhat (normal graphical station)
--- Downloaded and untared gentoo stage 1 to the root directory (I assume it overwrote most of the redhat stuff)
--- Ran the installation steps as per the x86 faq (no sparc faq at the time)
--- Didn't copy the image over correctly and failed on the reboot (my error)

Third time:
--- (last night infact) Installed redhat (custom again)
--- Followed faq
--- Got the masked ebuilds comment the other thread mentions
--- Ignored it and went through all of the steps with no other errors
--- Got one error when I did the ln with the vmlinuz from /boot to / (something about crossing devices and hard link)
--- Worked around and rebooted
--- Got error on restart about being unable to mount... please enter root password cus we can't start otherwise (What is the root password?)

At some point I'll try it on a different disk (machine has 2 disks. Had been spreading the redhat across both of them and hoping gentoo would use the same partition table..... Might have to change that.)